Fiber reinforcement of a ceramic composite offers significant opportunities for toughening the brittle matrix of the composite. There is an interest in fiber reinforced composites for use in high temperature applications, such as combustors and engines. Structures with carbon fiber reinforced carbon matrices have been tried but they have the disadvantage of poor oxidation resistance at high temperatures, such as 1200.degree. C. or above. High strength carbon fibers have also been tape cast in a slurry and formed into a preform and then infiltrated with molten silicon with the hope that the silicon matrix would protect the carbon filaments in high temperature, oxidative environments. However, in this process the carbon filaments tend to convert into relatively weak, irregular columns of silicon carbide crystals resulting in composites with low toughness and relatively modest strength.
To alleviate the problems with carbon fibers in a carbon matrix, large diameter (about 140 micrometers or greater) silicon carbide-containing fibers provided with a coating of boron nitride have been tried with molten silicon infiltration to form a silicon-silicon carbide composite. The silicon melt approach in combination with the silicon carbide material has proven to be effective in preventing damage to the fiber. The spacing of the fibers in the preform due to the large diameter size allows the molten silicon to react with the carbonaceous material in the preform to provide a silicon-silicon carbide matrix. However, the size of the fibers prevents the formation of complex shapes due to limited bend radii. When smaller discontinuous fibers are used (about 0.3 to 50 micrometers in diameter), a silicon-wetting coating is required over the boron nitride coating. Even then, if the small diameter fibers are bundled, they often do not wet with molten silicon and as a result, pockets of elemental silicon form in the silicon carbide matrix. This results in a weaker silicon carbide composite.
U.S. Pat. Nos. 5,015,540; 5,330,854; and 5,336,350; incorporated herein by reference, relate to the production of silicon carbide matrix composites containing fibrous material that is infiltrated with molten silicon, herein referred to as the Silcomp process. The fibers generally have diameters of about 140 micrometers or greater, which prevents intricate, complex shapes to be manufactured during formation of the preform. Limited bend radii of these fibers restrict their use to structures with radii of curvature between 1-2 inches in diameter. At smaller radii of curvature, the large diameter fibers will fracture at elevated temperatures of about 2000.degree. F. or greater due to the high residual stresses imposed in the fiber. Thus, there is a need for a method to make complex shaped preforms that incorporate continuous bundles of smaller diameter fibers in the preform that will subsequently provide tougher, stronger high temperature composites.
Presently, in the manufacture of silicon-silicon carbide composites, the fibers in the preform are coated with a low char yield slurry composition containing polymers that decompose upon heating. The polymers produce little or no char after decomposition, which means that there is little or no solid material after burnout. As a result, a low char yield slurry composition used to coat the fiber provides a low strength preform after burn-out processing. This is not desirable if the preform has to be moved or transported from one furnace to another. A need is created for an improved preform with high strength after burn-out.
The Silcomp process for making silicon-silicon carbide composite, uses coarse carbon and silicon carbide powders as filler materials in the slurry composition that is coated on the fibers or in the preform itself. The coarse powders do not completely react during the molten silicon infiltration to convert all of the available carbon to silicon carbide, thus yielding a high residual carbon content in the matrix (about 10-20 volume percent silicon). This creates a need for a lower elemental carbon in the silicon-silicon carbide composite.
Although operable, the above-mentioned Silcomp process provides an opportunity for process and product improvement. There is a need for complex shaped preforms that accommodate small diameter silicon carbide-containing fibers that are bundled and gathered in tows. There is also a need for a novel slurry composition to coat the small diameter fibers with high char yield resins to provide a stronger and tougher preform after burnout and containing fine particles of carbon and silicon carbide that can penetrate between the fibers that are bundled in tows. There is also a need for a method of making complex shaped preforms and composites by molten silicon infiltration that provides an improved silicon-silicon carbide composite.